Double rounded corner
You may use the Bridge Edge Loops option. The subsurfed result'll be similar to the one achieved with beveling.


Select the same amount of vertices laying on the oposite sides, press Spacebar, type Bridge and click this option. In the Tool Shelf options (T) change the number of cuts to fit the edge loops' number. Then unselect the top vertices (C, hold Mouse Wheel) and select only bottom ones (C). Hit W-->Remove Doubles and increase the merge distance. Fill the hole with diamond quads (select vertices and press F).